1. Field of the Invention
The present invention relates, in general, to a gas target for producing gas isotopes such as C-11 and, more particularly, to a radioisotope production gas target, in which a fin structure is formed in an internal space, i.e. a target cavity, in which stable isotopes that are target materials cause a nuclear reaction with protons, thereby stably and remarkably increasing a yield of the production of the isotopes.
2. Description of the Related Art
Generally, isotopes are produced by irradiating protons or neutrons to stable isotopes. In this manner, a mechanism or an apparatus that makes it possible to irradiate the protons or the neutrons to the stable isotopes refers to a target.
A radioactive medicament called 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) (hereinafter, referred to as “FDG”) that synthesizes fluorine (F) into glucose is used in positron emission tomography (PET) for the diagnosis of tumors or cancer. In the case of image diagnosis of a brain or a heart, gas isotopes such as C-11 are used for high reliability. The representative gas isotope, C-11, is converted into a radioactive compound such as methyl iodide (MeI) or acetate, and then is used for diagnosis.
The gas isotopes such as C-11 are produced by irradiating accelerated protons to gaseous stable isotopes. An apparatus for accelerating the protons is an accelerator called a cyclotron, which is widely used for research and diagnosis in many institutes which use the PET.
The gas target is basically configured of a target window that is an entrance into which the protons accelerated by the cyclotron are sent, a target cavity that is a space in which the accelerated protons cause a nuclear reaction with the target materials (or stable isotopes) so as to produce radioisotopes, a target cooling system that collects heat generated by energy absorption at the target cavity, and a targetry system that supplies the stable isotopes to the target and collects the produced radioisotopes.
The gas isotopes, C-11, which are to be used in the PET are produced through a nuclear reaction, 14N(p,α)11C, by generating a beam of protons from the accelerator, that is, the cyclotron, and irradiating the protons generated from the cyclotron to the stable isotopes, N2, that are the target materials.
The protons accelerated by the cyclotron are characterized in that the energy thereof is sharply reduced according to the density of material. Thus, the target window, which is a target incident section for producing the isotopes, is designed to least have only a mechanism so as to be able to maintain the proton energy at the maximum extent. For this reason, a thin metal sheet is used in the front of the target window through which the proton beam passes, and a structure such as a grid structure is installed together so as to be able to withstand high pressure.
FIG. 1 illustrates one example of a conventional gas target that is designed and used according to the aforementioned principle and basic configuration. A target window 10 onto which the proton beam is incident has a diameter of about 20 mm, which is designed to an appropriate size so that the proton beam can pass through when the proton beam generated from the cyclotron is widened to the maximum extent. A support structure 12 is installed adjacent to the target window 10 so as to support a thin metal sheet 14.
The gas target used for producing the isotopes is divided into two types, a cylindrical type and a conical type, according to a shape thereof. The conical type gas target is adapted to the spatial shape of proton beam locus increasing its cross section by scattering as it approaches the second half thereof in the gas target (see FIG. 2).
A portion where the nuclear reaction is produced by the proton beam undergoes a phenomenon called density reduction caused by compressibility of gas as well as generation of heat. Here, the density reduction refers to an effect where the heat is generated from portion where the nuclear reaction occurs by the application of the proton beam, and thus the portion where the nuclear reaction occurs is subjected to a reduction in density, whereas a surrounding portion remote from the portion where the nuclear reaction occurs is subjected to an increase in density. For this reason, a length of the proton beam passing through the gas is varied, and secondary beam scattering takes place at a rear end where the nuclear reaction occurs (The International Journal of Applied Radiation and Isotopes, Volume 33, Issue 8, August 1982, Pages 653-659, Sven-Johan Heselius, Peter Lindblom, Olof Solin; The International Journal of Applied Radiation and Isotopes, Volume 35, Issue 10, October 1984, Pages 977-980, Sven-Johan Heselius, Peter Lindblom, Ebbe M. Nyman, Olof Solin).
Further, when beam divergence is larger than the diameter of an interior of the target, i.e. a target cavity, according to a characteristic of the proton beam, this leads to a loss of the energy of the proton beam, and thus serves as a factor that reduces production yield of radioisotopes. Accordingly, the loss of the proton beam energy is prevented by a conical gas target, which has been recently manufactured according to a shape corresponding to a shape of the beam divergence. Thereby, the conical gas target is being studied beginning from the concept that the conical gas target obtains a yield higher than that of the cylindrical gas target.
However, the production of the radioisotopes using the cylindrical or conical gas target is basically accompanied with a generation of high pressure, so that it causes a problem with safety of the thin metal sheet installed as the target window. Further, such production fails to effectively inhibit the effect of the density reduction caused by the nuclear reaction, so that it increases instability of the production yield. In other words, only the conversion of the shape of the gas target from the cylindrical type to the conical type has a limitation to improving the production yield of the radioisotopes and maintaining production stability of the radioisotopes.
In order to ensure a stable production yield of the radioisotopes, it is necessary to effectively cool the gas target. Thus, the conventional gas targets as illustrated in FIG. 1 have employed a method of lowering a temperature of a coolant flowing through a cooling channel 18 installed outside a target chamber 16 in order to inhibit the gas in the target chamber 16 from being raised in temperature, or a method of increasing a heat transfer area by forming cooling fins (not shown) on an outer surface of the target chamber 16 that is in contact with the coolant.
However, the cooling fins are based on a basic concept that the cooling fins are installed when heat exchange and heat transfer effects of a fluid can be expected to be improved by increasing a heat radiation surface area in a direction in which the heat transfer from the fluid does not sufficiently occur. As such, the configuration in which the cooling fins are formed on the outer surface of the target cavity as in the conventional gas target is estimated to be not quite optimal.
In other words, when the cooling fins are formed on the outer surface of the target cavity, the outer surface of the target cavity that is in contact with the coolant may be sufficiently cooled. However, since the capacity of heat transmitted from the target gas of the target cavity which is generally no more than one several hundredth of the liquid to the outer surface of the target cavity is not sufficient, it will be difficult to expect the cooling of the target gas. As such, it is necessary to design the gas target based on a new concept so that the target material, i.e. the gas, in the target cavity itself can be effectively cooled.